1. Field of the Invention
The present invention generally relates to handheld lighting units and more particularly to handheld fluorescent lighting units having an improved electronic ballast, enhanced forward illumination, resistance to mechanical impact, and accommodation of one or more of various types of fluorescent bulbs.
2. Description of the Prior Art
Portable, hand-held drop lights or task lamps utilizing an incandescent bulb and powered by AC line current, typically 120 Volts AC, 60 Hz, allow the user to provide light where installed light fixtures do not provide adequate coverage. However, incandescent bulbs as the light source in task lamps have several disadvantages. It is well known that incandescent light bulbs are not economical to operate because much of the electrical energy used by the task light is converted to heat. The tungsten filament in a typical 100 Watt incandescent bulb causes the bulb to get too hot to touch, or even use close to one's person. Moreover, the relatively fragile nature of the tungsten filament impairs the utility of a task lamp in many work situations.
One alternative to the use of incandescent bulbs is the fluorescent bulb. Fluorescent bulbs convert more of the supplied electrical energy to light energy and radiate much less heat than do incandescent lights. The light emitting medium in fluorescent lights is a phosphor coating, unlike the thin, fragile tungsten filament in an incandescent light bulb. In a fluorescent lamp bulb, a glass tube containing a small amount of gas—mercury vapor, for example—is provided with coated cathode electrodes at either end of the tube. When a high enough voltage is applied between each pair of electrodes at the ends of the glass tube, the coated filament is heated and emits electrons into the gas inside the tube. The gas becomes partially ionized and undergoes a phase change to a plasma state. The plasma is conductive and permits an electric arc to be established between the electrodes. As current flows in the plasma, electrons collide with gas molecules, boosting the electrons to a higher energy level. This higher energy level is not a stable condition and when the electron falls back to its normal energy level, a photon of ultra-violet light is emitted. The photons in turn collide with the phosphor coating on the inside of the glass tube, imparting their energy to the phosphor ions, causing them to glow in the visible spectrum. Thus the phosphor coating luminesces and gives off the characteristic “fluorescent” light.
However, fluorescent bulbs require a relatively high voltage to initiate the plasma state. After the plasma state is initiated, i.e., the bulb is ignited, the effective resistance of the plasma between the electrodes drops due to the negative resistance characteristic of the fluorescent bulb. Unless the current is limited after ignition of the bulb, the tube will draw excessive current and damage itself and/or the supply circuit. The dual functions of igniting the fluorescent bulb and limiting the current in the bulb after ignition takes place are performed by a ballast circuit. The ballast for full-sized installed light fixtures includes a large transformer/inductor, to transform the supplied line voltage, typically 120 Volts AC available at a wall outlet to a high enough potential to ignite the lamp and also to provide a high enough inductive impedance in the supply circuit to limit the current during operation. For typical installed lighting fixtures using non-self-starting bulbs and operating at 120 VAC, 60 Hz, the wire gauge, the number of turns in the coils, and size of the magnetic core result in a large and heavy ballast component. The ballast circuits for so-called “self-starting” fluorescent bulbs are typically smaller, yet still provide an appropriate voltage to ignite the lamps without a separate starter. The inductive impedance of the ballast circuit then regulates the current draw in a similar manner to that previously described for non-self starting bulbs.
In recent years electronic ballast circuits have been developed to replace the large inductors used in the traditional fluorescent lamp ballasts. The electronic ballasts are much lighter in weight because they operate at much higher frequencies and thus have much smaller inductive components. Such “solid state” ballasts are also very efficient and can be manufactured at low cost, making them especially suited for use in small, handheld fluorescent lamps. In one example of the prior art, U.S. Pat. No. 6,534,926, Miller et al., a portable fluorescent drop light is disclosed that contains a pair of twin-tube compact fluorescent lamp (CFL) bulbs that are individually switched. The discrete solid state drive circuit used as a ballast for non-self-starting bulbs utilizes the CFL bulbs as part of the oscillating circuit and has a relatively high component count. A different ballast circuit is required for use with self-starting bulbs. Miller et al. thus has the disadvantages of relatively high component count, and is not capable of driving non-self-starting or self-starting bulbs from the same ballast circuit. Further, while the output from the two 13 Watt CFL bulbs provides adequate illumination, the diffuse light is radiated into all directions and is not controlled or directed in any way so as to maximize the utility of the illumination for task lighting. The portable fluorescent lamp disclosed by Miller et al. further appears to lack the ability to withstand mechanical impacts that frequently occur during the use of task lamps.
A need exists, therefore, for an economical, portable hand-held task lamp that provides a light output substantially equivalent to that of a 100 Watt incandescent bulb, is efficient to operate, and does not operate at excessively high temperatures. A need also exists for a cool-running, efficient task lamp that provides an enhanced illumination output, directing the available light toward the task being illuminated. A need also exists for a ballast circuit design that can accommodate and operate with either self-starting or non-self-starting bulbs, can start and run whether one or both bulbs are installed in the task lamp, and does not require separate switches or separate circuits to operate two or more bulbs. The lamp should further be resistant to damage from mechanical impact and utilize inexpensive, readily available fluorescent bulbs. It would be a further desirable feature to provide as light-weight and compact a task lamp as possible.